This invention relates to methods and apparatus for supercritical fluid processing of substrates. More particularly, it relates to methods and apparatus for maintaining wafer back side, bevel, and front side edge exclusion during supercritical fluid processing.
Wafer handling in modern VSLI semiconductor processing presents numerous engineering dilemmas. One particularly important issue involves processing the wafer""s work surface while protecting its back side and edge bevel from contamination with the processing medium. Typical processing operations include: deposition, etch, or other modification of the wafer work surface. Metal contamination on a wafer back side and edge bevel is especially problematic. Metal contaminates ultimately end up on the front side, due to handling, ultimately ruining devices on the front side due to diffusion of metal into unprotected dielectrics.
Key components in wafer handling during processing are wafer chuck and pedestal assemblies as well as end effectors. End effectors are used to deliver wafers to and from wafer pedestals. Preferably, end effectors grab wafers only on the back side to avoid contamination or alteration of the work surface. The wafer back side is typically in contact with the pedestal, held in place by a restraining device, for example a vacuum, electrostatic, or multi-point clamp. Clamps must hold the wafer in place while avoiding application of undue stress to the wafer. Additionally, it is often desired that the wafer make good thermal contact to the pedestal so that pedestal-mediated heating or cooling processes are efficient.
Cleary et al. (U.S. Pat. No. 6,281,144) and van de Ven et al. (U.S. Pat. No. 5,882,417) describe methods and apparatus for back side and edge exclusion during CVD processing. In these inventions, unique pedestal design with controlled gas flow patterns provide protection of the back side and edge bevel regions by blocking deposition of materials on these areas during processing. While these inventions are effective with respect to low pressure CVD processing conditions, they do not address the unique conditions encountered during wafer processing with supercritical fluids.
Under supercritical process conditions, there are very large pressure changes, density gradients, and temperature effects. For example, convection in supercritical fluids may transfer heat from a wafer too quickly. Drastic pressure changes can shatter wafers having even the slightest stress due to the restraining device. Additionally, movement of mechanical apparatus is restricted under supercritical pressure. Preferably, supercritical reactors have minimal moving parts. Thus supercritical process conditions present unique challenges for design of wafer handling apparatus and methods for using them.
What is therefore needed are improved apparatus and methods for handling wafers in supercritical fluid reactors. In particular, what is needed are apparatus and methods that provide wafer back side, bevel, and front side edge exclusion during supercritical fluid processing, preferably apparatus and methods that take advantage of supercritical fluid process conditions.
The present invention pertains to apparatus and methods for maintaining wafer back side, bevel, and front side edge exclusion during supercritical fluid processing. Apparatus of the invention include a pedestal and an exclusion ring. When the exclusion ring is engaged with the pedestal, a channel is formed. A reactant-free supercritical fluid is passed through the channel and over a circumferential front edge of a wafer. The flow of reactant-free supercritical fluid protects the bevel and a circumferential front edge of the wafer from exposure to reactants in a supercritical processing medium. The back side of the wafer is protected by contact with the pedestal and the flow of reactant-free supercritical fluid.
One aspect of the invention is an apparatus for immobilizing a wafer and preventing a reactant from contacting the back side, bevel, and a circumferential front edge of the wafer during a surface modification process on the front side of the wafer in a supercritical medium containing the reactant. Such apparatus may be characterized by the following features: (a) a pedestal having a wafer support surface, the wafer support surface configured to support the wafer via the wafer back side; and (b) an exclusion ring which when engaged with the pedestal, is configured to provide a flow of a reactant-free supercritical fluid and prevent the reactant in the supercritical medium from contacting the back side, bevel, and the circumferential front edge of the wafer during the surface modification process. Preferably the apparatus is part of a supercritical reactor assembly. Also preferably the circumferential front edge is between about 0.1 mm and 5 mm wide, more preferably between about 1 mm and 3 mm wide. Each of the reactant-free supercritical fluid and the supercritical medium preferably include at least one of carbon dioxide, ammonia, water, ethanol, ethane, propane, butane, pentane, dimethyl ether, hexafluoroethane, and mixtures thereof.
Preferably the pedestal has an internal heat source and the wafer support surface has a lip configured to center the wafer on the wafer support surface. Preferably the lip protrudes from the wafer support surface at least to the elevation of the front side of the wafer when the wafer is on the wafer support surface. Also preferably, the lip has a sloped surface configured to engage the wafer edge.
In preferred embodiments, the pedestal has a flange with a mating surface configured to mate with the exclusion ring when the pedestal and the exclusion ring are engaged. Preferably the mating surface has an elastomeric seal, configured to form a fluid-tight seal between the pedestal and the exclusion ring during engagement. Preferably the elastomeric seal comprises at least one of an O-ring, a cup seal, a flap seal, a T-seal, a C-seal, a self-energized seal, and the like. Also preferably the elastomeric seal comprises at least one of Viton, Kalrez, Chemraz, silicone, perfluorinated hydrocarbons, EPDM, PTFA, and the like.
Also, the exclusion ring and the pedestal form a channel when engaged. The channel is configured to provide the flow of the reactant-free supercritical fluid to the wafer. The reactant-free supercritical fluid exits the channel through a channel outlet between about 25 and 250 xcexcm wide, more preferably about 50 xcexcm wide. Preferably the flange has a plurality of apertures configured to supply the flow of the reactant-free supercritical fluid to the channel. Preferably the flow rate of reactant-free supercritical fluid exiting the channel outlet is between about 0.5 and 50 cc per minute, more preferably between about 5 and 20 cc per minute. In some embodiments the exclusion ring is configured to divert a portion of the flow of reactant-free supercritical fluid to a location other than the wafer surface. Preferably the portion diverted is between about 50 and 95% of the total flow of reactant-free supercritical fluid, more preferably between about 70 and 90%.
In some preferred embodiments, the exclusion ring has an overhang that comprises part of the channel. The overhang is configured to protrude over the wafer""s front edge when the exclusion ring is engaged with the pedestal. Preferably the overhang protrudes between about 1 mm and 10 mm over the wafer""s front edge, more preferably between about 1 mm and 5 mm.
Exclusion rings of the invention, when engaged with their corresponding pedestals make no or very little physical contact with the wafer front side. A non-contact exclusion ring is used when no contact with the wafer front side is desired. In some preferred embodiments a contact exclusion ring is used. Contact exclusion rings of the invention preferably have minimal contact with the wafer front side.
Mechanisms for delivering the wafer to the pedestal are embodied in three exemplary apparatus. In one apparatus, the pedestal comprises a plurality of active pins arranged to receive the wafer via the wafer back side and recess into the wafer support surface of the pedestal in order to deliver the wafer to the wafer support surface. Preferably such an apparatus is used with a non-contact exclusion ring. In a second apparatus, the exclusion ring is a non-contact exclusion ring comprising a plurality of wafer transfer fingers arranged to support the wafer via the wafer backside for delivery of the wafer to the wafer support surface. In a third apparatus, the pedestal is an inverted pedestal, and the exclusion ring is a contact exclusion ring having a plurality of point contacts arranged to support the wafer via the wafer front side for delivery of the wafer to the wafer support surface. In this third example, the contact exclusion ring, when engaged with the inverted pedestal, is also configured to hold the wafer onto the wafer support surface of the inverted pedestal via the plurality of point contacts. The plurality of point contacts are configured to exert a force opposing gravity on the front side of the wafer.
Another aspect of the invention is a method of conducting a surface modification process on the front side of a wafer in a supercritical medium containing a reactant without allowing the reactant to contact the back side and a circumferential front edge of the wafer. Such methods may be characterized by the following sequence: (a) providing the wafer to a reactor having a pedestal with a wafer support surface, the back side of the wafer contacting the wafer support surface; (b) charging the reactor with a reactant-free supercritical fluid; (c) providing a flow of the reactant-free supercritical fluid over at least the circumferential front edge of the wafer in a manner that prevents the reactant in the supercritical medium from contacting the circumferential front edge, bevel, and back side of the wafer; (d) providing the supercritical medium to the reactor; (e) performing the surface modification process; and (f) flushing the reactor with the reactant-free supercritical fluid.
Methods of providing the wafer to a reactor, as described in (a) above, depend upon the particular apparatus used. With respect to apparatus of the invention having non-contact exclusion rings and pedestals with active pins, (a) may be characterized by the following sequence: (i) raising a plurality of active pins from a recessed position in the pedestal so that they protrude from the wafer support surface; (ii) placing the wafer on the plurality of active pins, the back side of the wafer resting on the plurality of active pins; and (iii) returning the plurality of active pins to the recessed position in the pedestal, thereby resting the back side of the wafer on the wafer support surface. Preferably the non-contact exclusion ring is engaged with the pedestal immediately after (a). With respect to apparatus of the invention having non-contact exclusion rings with wafer transfer fingers and pedestals with recesses that mate with the transfer fingers, (a) may be characterized by the following sequence: (i) inserting the wafer into a non-contact exclusion ring having a plurality of wafer transfer fingers arranged to support the wafer via the wafer backside; and (ii) engaging the non-contact exclusion ring with the pedestal, during engagement, the plurality of wafer transfer fingers travel into a plurality of corresponding recesses in the pedestal and transfer the wafer to the wafer support surface of the pedestal. With respect to apparatus of the invention having contact exclusion rings and inverted pedestals, (a) may be characterized by the following sequence: (i) inserting the wafer into a contact exclusion ring having a plurality of point contacts arranged to support the wafer via the wafer front side; and (ii) engaging the contact exclusion ring with the inverted pedestal, the plurality of point contacts holding the wafer against the wafer support surface of the inverted pedestal via the wafer front side. Preferably, exclusion rings of the invention, when engaged with their corresponding pedestals, are configured to provide the flow of reactant-free supercritical fluid as described in (c) above.
These and other features and advantages of the present invention will be described in more detail below with reference to the associated drawings.